diff --git a/jaxrs/src/main/java/org/killbill/billing/jaxrs/resources/PaymentResource.java b/jaxrs/src/main/java/org/killbill/billing/jaxrs/resources/PaymentResource.java
index 3a61d7f..822a100 100644
--- a/jaxrs/src/main/java/org/killbill/billing/jaxrs/resources/PaymentResource.java
+++ b/jaxrs/src/main/java/org/killbill/billing/jaxrs/resources/PaymentResource.java
@@ -253,7 +253,8 @@ public class PaymentResource extends JaxRsResourceBase {
final Account account = accountUserApi.getAccountById(initialPayment.getAccountId(), callContext);
- final Payment payment = paymentApi.createVoid(account, paymentId, json.getTransactionExternalKey(), pluginProperties, callContext);
+ final String transactionExternalKey = json != null ? json.getTransactionExternalKey() : null;
+ final Payment payment = paymentApi.createVoid(account, paymentId, transactionExternalKey, pluginProperties, callContext);
return uriBuilder.buildResponse(uriInfo, PaymentResource.class, "getPayment", payment.getId());
}